1. Field of the Invention
The present invention relates to devices that are implanted in animals, for example, human beings, and have direct contact with blood.
2. Description of the Prior Art
In certain devices implantable in an animal, such as a human, it is desirable to provide blood-contacting surfaces that are substantially stable and biocompatible. One of the problems that can be encountered in connection with implanted devices is incompatibility of blood-contacting surfaces with the blood to the extent of inducing thrombosis or damage to the blood.
One approach to producing hemocompatible surfaces is to bond polyester fibrils to polyurethane substrates. The bonded fibrils or flocking can create a textured surface that can initiate controlled blood clotting which ultimately can result in a generally stable biological lining. Although a textured surface of polyester flocking can promote formation of a biologic lining, fibers may separate from the substrate and can be washed away in the blood stream to lodge in vital organs such as the brain or spleen.
U.S. Pat. No. 4,016,303 discloses a method for addressing the separation of polyester fibers from blood pump surfaces by over-coating the fibers with polyurethane after bonding the fibers to the polyurethane substrate. However, fiber separation can still occur and implantable devices produced by that method may be unacceptable for particular applications.
Another approach to forming a textured, biocompatible surface can involve forming the fibers as an integral part of the blood-contacting devices, such as a polyurethane blood bag, so that an adhesive bond between fibers and substrate may not be used. By forming the blood bag on a mold that had a surface containing millions of microscopic pores or invaginations, such textured surfaces can be obtained. Using this method, a female mold of the desired device shape can be constructed, typically of brass, and assembled. The female mold can be placed in the tail stock of a lathe and rotated while a custom-built electrostatic applicator is inserted into the mold cavity to apply nylon flock to the mold surface. A thermoset silicone molding compound can be cast into the mold cavity to make a male mold for dip-coating, or solution casting, of the desired substrate material. The nylon fibers can be dissolved from the silicone male mold with a solvent solution such as phenol-methylene chloride to produce a mold that can contain millions of microscopic invaginations which correspond to the dissolved nylon fibers.
A significant limitation of this technique is that it accommodates device configurations of relatively simple shape, primarily axi-symmetric structures such as a cylinder because the electrostatic applicator needs to fit inside the rotating mold assembly. The size of the electrostatic applicator also can impose a practical minimum diameter for the blood bag, which can be dictated by the diameter of the applicator and the air gap between applicator and mold which can prevent electrical arcing. Another limitation of this method can be that the cross-sectional area of the blood bag may not vary significantly because of the difficulty in stretching the polyurethane bag to remove it from the silicone male mold.
What is needed then is an implant having integrally-textured, hemocompatible blood-contacting surfaces, and a method for producing the same, that obviate the aforementioned limitations and disadvantages. Moreover, it is desired to produce such implants having complex shapes.